1. Field of the Invention
The present invention relates to a process and apparatus for forming a fluorescent layer on a substrate for display panel. More particularly, the present invention relates to a process for forming a fluorescent layer on a substrate for display panel such as a plasma display panel (PDP) of matrix display type.
2. Description of the Related Arts
A PDP is a thin display device excellent in visibility, capable of high speed display, and facilitating a large screen display. Especially, the PDP of surface discharge type is a PDP in which display electrodes forming a pair in applying a driving voltage are arranged on the same substrate. Such a PDP is suitable for full color display using fluorescent substances.
Conventional PDP is now explained with reference to a PDP of AC type employing a three-electrode surface discharge method capable of full color display. FIG. 18 is an exploded perspective view of a conventional PDP, which shows a portion corresponding to one pixel EG.
A PDP is constructed with a glass substrate 11 on a front side and a glass substrate 21 on a back side. First and second display electrodes X and Y, a dielectric layer 17, and a protective film 18 are formed on the glass substrate 11 of the front side. Address electrodes A, a rib (barrier rib) 26, a fluorescent layer 28R (for red), a fluorescent layer 28G (for green), and a fluorescent layer 28B (for blue) are formed on the glass substrate 21 of the back side. A discharge gas is injected in a discharge space 30 between the glass substrate 11 of the front side and the glass substrate 21 of the back side.
Each of the display electrodes X and Y is formed of a wide transparent electrode 41 and a narrow metal electrode (bus electrode) 42 for compensating for the conductivity of the transparent electrode 41.
Application of a predetermined voltage to the display electrodes X and Y induces surface discharge along the surface of the dielectric layer 17, whereby ultraviolet light emitted by the discharge gas excites the fluorescent layers 28R, 28G, and 28B to emit a visible light. The address electrodes A are arranged in a column direction for controlling the wall charge of the dielectric layer 17 by generating a discharge between the address electrodes A and the display electrode Y.
The ribs are stripe-like in a plan view (seen in a plane) and equally spaced along the extending direction of the display electrodes X and Y (in a row direction of the display screen). These ribs 26 partition the discharge space 30 into unit display elements (subpixels) EU in the row direction and allow the gap dimension of the discharge space 30 to be uniform over the entire display region.
Referring to the PDP shown in FIG. 18, the pixel EG constituting the screen consists of three subpixels EU of R (red), G (green), and B (blue) arranged in a row (line) direction. In other words, the arrangement of three colors for color display is of in-line type.
Conventionally, the fluorescent layers 28R, 28G, and 28B were formed by applying paste-like fluorescent substances (fluorescent pastes) between the ribs 26 for the three colors, followed by drying and calcination. The application of the fluorescent pastes was carried out by screen printing method.
In the screen printing method, the printing is carried out three times by using three printing plates for R, G, and B. In other words, a fluorescent paste for R is applied between the ribs 26 for R by using a plate for R before drying, then for G, and finally for B. Thus, the process of aligning the printing plate with the glass substrate 21 of the back side having the ribs thereon for applying and drying the fluorescent paste was repeated three times before the calcination step.
Such a screen printing method will require printing plates of highly fine structure and wide area according as the display panel has a highly fine structure and wide area. Since the printing plates of highly fine structure and wide area tend to elongate or bend, it is difficult to align the printing plates with the glass plate 21 of the back side. Therefore, a problem arises such that the fluorescent paste is not applied on a desired unit display region (between the ribs), causing blotting of colors and partial insufficient application of the colors.